Methods for forming a thin film on the surface of a substance using a gaseous phase in which a substance is allowed to move at an atomic or molecular level in a similar manner to gas are known. These methods include chemical vapor deposition (CVD) (hereinafter, referred to as “CVD”) and physical vapor deposition (PVD) (hereinafter, referred to as “PVD”).
PVD includes, for example, vacuum vapor deposition and sputtering. Sputtering, which enables production of a high-quality thin film with a uniform film quality and thickness, has been widely applied to display devices such as liquid crystal displays.
CVD is a method of growing a solid thin film by introducing raw material gas into a vacuum chamber and decomposing or reacting one or two types of gas on a substrate by means of heat energy.
Here, a plasma or catalyst reaction is commonly used in order to promote the reaction during film formation or decrease the reaction temperature.
CVD using plasma reaction is called plasma enhanced CVD (PECVD), and CVD using catalyst reaction is called Cat-CVD.
These CVD methods can reduce deficiencies in film formation, and are applied to a manufacturing process of semiconductor devices (for example, formation of a gate insulating film) or the like.
In recent years, atomic layer deposition (ALD) (hereinafter, referred to as “ALD”) has attracted attention.
ALD is a method in which a substance adsorbed on a surface is deposited layer by layer at an atomic level by means of chemical reaction on the surface. ALD is classified as a type of CVD.
A typical CVD (a generally known CVD) is a method of growing a thin film by reacting a single gas or a plurality of gases simultaneously on a substrate. In contrast, ALD is a specific method of deposition for growing thin films at an atomic level layer by layer by alternately using a highly active gas or precursor (such as tri-methyl aluminum (TMA); hereinafter, “first precursor”) and a reactive gas (in ALD, also called a precursor; hereinafter, “second precursor”) by means of adsorption on the substance surface and subsequent chemical reaction.
Specifically, ALD processes are conducted as follows.
First, when precursors have been adsorbed onto the substrate to form only one layer, unreacted precursors are purged by using a so-called self-limiting effect (a phenomenon during a surface adsorption on the substrate that gas is no longer adsorbed onto a surface when the surface is covered by a certain type of gas) (first step).
Then, a reactive gas is introduced into a chamber to oxidize or reduce the above precursors to thereby form only one layer of a thin film having a desired composition, and after that, the reactive gas is purged (second step).
In ALD, the above first and second steps are taken as one cycle, which is repeated to grow thin films on the substrate.
Accordingly, ALD grows thin films in two dimensions. Further, compared with the conventional vacuum vapor deposition or sputtering as well as the conventional CVD, ALD is characterized in reducing deficiencies in film deposition.
Accordingly, ALD is expected to be widely applied to the packaging field for foods, pharmaceutical products, or the like, or the electronics field.
PTL 1 discloses a product comprising a substrate made of a material selected from the group consisting of a plastic and a glass, and a gas transmission barrier deposited on the substrate by atomic layer vapor deposition.
Further, PTL 1 discloses that a light emitting polymer is mounted on a plastic substrate having optical transparency and atomic layer vapor deposition is performed on a top and side surfaces of the light emitting polymer by means of ALD (top coating), thereby achieving reduction in coating deficiencies and drastically reducing gas permeability for the thickness of several tens of nanometers.
Formation of the atomic layer deposition film on the substrate made of a polymer material by means of ALD is considered probably different in the form of growth from formation on a substrate made of an inorganic crystal such as Si wafer.
When an atomic layer deposition film is formed on the substrate by means of ALD by using the substrate of Si wafer having an oxidation treated surface, adsorption sites for precursors which serve as raw materials of the atomic layer deposition film are present at substantially the same density as the lattice of crystal, and growth of the film proceeds in a two-dimensional growth mode.
On the other hand, when an atomic layer deposition film is formed on the substrate made of a polymer material by means of ALD, it is known that adsorption sites for precursors which serve as raw materials of the atomic layer deposition film are present at a low distribution density, which results in the adsorption sites growing in three dimensions around nucleuses, which are the precursors adsorbed separated from each other, which causes contact between the adjacent nucleuses, leading to formation of a continuous layer.
Further, depending on the state of the substrate made of a polymer material and the ALD process conditions, growth into a columnar shape from the outer surface of the substrate in a direction perpendicular to the outer surface of the substrate is likely to occur.
That is, when the atomic layer deposition film is formed on the substrate made of a polymer material by means of ALD, there is a risk that gas may pass through gaps between a plurality of columnar structures that constitute the atomic layer deposition film from the outer surface of the atomic layer deposition film toward the substrate.
In other words, when the atomic layer deposition film is formed on the substrate made of a polymer material by means of ALD, there is a risk that the atomic layer deposition film will not have desired gas barrier properties.
A technique for solving the above problem is disclosed in PTL 2.
PTL 2 discloses formation of an undercoat layer containing an organic polymer by using an organic binder containing an inorganic substance on a substrate (outer surface of the substrate) made of a polymer material.